Fix external domain tls tests

[dprotaso]

• Fix external domain tls tests - bad substitution
• fix env var parsing

[dprotaso]

/test all

[dprotaso]

/test https

[knative-prow[bot]]

@dprotaso: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
https_serving_main	fd154494897d1f92a846e2d804f9e0c5229d377d	link	false	/test https

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[dprotaso]

/assign @ReToCode

This doesn't fix the tests but fixes the setup

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, ReToCode

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